1. Field of the Invention
The invention relates to a gas-turbine blade and to a method of manufacturing a gas-turbine blade.
Gas-turbine blades comprising a hollow, supporting metal blade body having a multiplicity of integrally cast or worked-in groove-shaped cooling passages, onto which a thin outer skin is applied by electron-beam vaporization or plasma spraying, are known, for example, from U.S. Pat. No. 5,640,767. The cooling effect of the air cooling is to be improved with these gas-turbine blades. The underlying premise is that the cooling effect becomes better and better as the thickness of the blade wall decreases. However, if the thickness of the supporting blade wall is reduced, this is accompanied by a marked loss in the strength of the component.
The application of a thin outer skin by electron-beam vaporization or plasma spraying is described in Superalloys 1996 under the title xe2x80x9cAdvanced Airfoil Fabricationxe2x80x9d, pages 523-29. That disclosure constitutes an improvement compared with attempts to cast a double-walled blade in which the inner, thick wall absorbs the force impacts and in which the outer, thin wall merely constitutes an aerodynamically favorable envelope which can be readily cooled. Such blades are costly to manufacture and do not enable a favorable ratio to be achieved between the thickness of the inner, supporting wall and the outer, thin wall, so that the improvement in the cooling effect is only slight.
The cooling effect achieved with the method described in the paper xe2x80x9cAdvanced Airfoil Fabricationxe2x80x9d and with the gas-turbine blades resulting therefrom is not optimal. This is due to the fact that the flow resistance in the groove-shaped cooling passages is high and only inner cooling takes place through these groove-shaped cooling passages. Such a turbine blade is also described in U.S. Pat. No. 5,640,767.
A cooled gas-turbine blade is described in U.S. Pat. No. 5,392,515. There, cooling pockets are provided on the outside of a wall enclosing a hollow space and are connected to the hollow space via passages for the cooling-air feed.
2. Summary of the Invention
The object of the invention is to provide a gas turbine blade and a method of manufacturing a turbine blade which overcomes the above-noted deficiencies and disadvantages of the prior art devices and methods of this kind, and which method is simple to carry out and with which a gas-turbine blade for use at high temperatures can be manufactured. It is a further object of the invention to specify a gas-turbine blade of appropriate configuration.
With the above and other objects in view there is provided, in accordance with the invention, a method of manufacturing a gas-turbine blade, which comprises the following steps:
casting a hollow, supporting metal blade body with a blade airfoil having an outer surface and a hollow interior space;
forming a multiplicity of elevations with top surfaces on the outer surface of the blade airfoil;
boring impact cooling bores between the elevations from the outer surface to the hollow interior space;
applying a coating of a heat-resistant, removable material and thereby filling intermediate spaces between the elevations and closing the impact cooling bores;
applying a covering coat adhering to the top sides of the (metallically bright) elevations; and
removing the material from the intermediate spaces between the elevations and the covering coat.
The invention is based on the idea that the cooling-air feed to the outer covering coat is to be as intensive as possible so that the gas-turbine blade manufactured according to the method can be operated at very high temperatures, i.e. at more than 1250xc2x0 C. This is achieved in that the covering coat is connected to the blade airfoil only in a spot-like manner via, in particular peg-like, elevations, so that there is a large, free intermediate space between the covering coat and the outer surface of the supporting metal blade body. Cooling air is directed into the intermediate space through the impact cooling bores, and the inner surface of the covering coat is intensively cooled by impact cooling.
The elevations are configured in particular in such a way that the width between two adjacent elevations is greater than the width of the elevation itself. In addition to peg-like elevations, elongated elevations or elevations of a different kind or even elevations of various shapes together on the blade airfoil are possible in a development. By suitable selection of the geometry of the elevation and of the distribution of the elevations over the blade airfoil, the heat flow over the latter can be set in a specific manner. In this case, the distribution of the elevations depends on stability criteria with regard to the covering coat to be applied.
In accordance with an added feature of the invention, the elevations are cast simultaneously during the casting of the metal blade body, i.e. they are cast along with the latter. This saves additional operations and permits an especially reliable connection between the blade airfoil and the individual elevations.
The supporting metal blade body having the elevations (e.g., peg-like elevations) can be manufactured as a casting in a highly precise manner, and the boring or drilling of the impact cooling bores can likewise be carried out in a simple manner, preferably by means of a laser beam.
The heat-resistant and removable material which fills the intermediate spaces is preferably a ceramic material, which is dried and sintered after the application. This ceramic material may be the material which is also used as core material during the casting of the hollow, supporting metal blade body. In particular, this material is also leachable.
In order to obtain reliable adhesion between the covering coat to be applied and the top sides of the elevations, the surface of the coated metal blade body, after the drying and sintering of the coating, may be machined by grinding, so that the top sides of the elevations are exposed in a metallically bright state.
The application of a metal covering coat may be effected in a vacuum by electron-beam vaporization, whereas covering-coat materials which may also be non-metallic can also be applied by plasma spraying. A metal covering coat is distinguished by the fact that it has good thermal conductivity and ensures an intermetallic connection with the metallically bright top sides of the elevations. An additional oxidation and corrosion inhibiting coat may also be applied to this metal covering coat.
The connection between the covering coat and the metallically bright top sides of the elevations of the blade body can be improved by a heat treatment. Diffusion processes, which produce a reliable, intermetallic connection between the covering coat and the metal blade body, are activated by the heat treatment.
To cool the outer surface, exposed to the hot gases, of the covering coat, the latter may be formed with a multiplicity of oblique film-cooling bores. During the operation of the component, the discharging cooling air then forms a cooling film which flows along the outer surface.
The outer surface of the blade can then be precision-machined and/or smoothed. After that, if need be, it may also be given a further ceramic coating, e.g. consisting of a ZrO2 coat, which is at least partly stabilized with yttrium. Such a ceramic coating may also be applied to the inner surface of the hollow, supporting metal blade body.
With the above and other objects in view there is also provided, in accordance with the invention, a gas-turbine blade, comprising:
a hollow, supporting metal blade body having a blade airfoil with an outer surface formed with a multiplicity of elevations;
the blade airfoil having a multiplicity of impact cooling bores formed therein between the elevations; and
a thin covering coat adhering to the top sides of the elevations.
The covering coat has a thickness of between 0.1 mm and 0.5 mm and/or the elevations are shaped as pegs.
This novel gas-turbine blade satisfies the objects of the invention, namely to solve the problem, mentioned above, concerning the high-temperature resistance and the ease of manufacture.
The covering coat may preferably be formed with a multiplicity of oblique film-cooling bores. The thickness of the covering coat is preferably between about 0.1 mm and 0.5 mm, in particular between 180 xcexcm and 300 xcexcm. Cooling air flowing through the impact cooling bores cools the inner surface of the covering coat by impact cooling. The cooling air discharges at least partly through the oblique film-cooling bores onto the outer surface of the covering coat and forms a cooling film. The impact regions of the impact cooling air are preferably offset from the inlet openings of the film-cooling bores, e.g. by between one quarter or half the spacing of the impact cooling bores. In this case, the covering coat may be made of metal and be provided, if need be, with an additional oxidation and corrosion coat, e.g. a so-called MCrAlY alloy. In addition to the outer surface, the inner surface of the hollow blade body may also be provided with a ceramic coating. The inner surface is preferably covered with a protective coat, in particular chromated or alitized. The elevations are preferably peg-like.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a gas-turbine blade and a method of manufacturing a gas-turbine blade, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.